In recent years, with the demand for new materials, concern over the supply of strategic elements, and the growing awareness of waste associated with corrosive and frictional forces, activity in laser alloying of materials has increased substantially.
A typical, well known technique for alloying materials together using a continuous beam of laser light is described in U.S. Pat. No. 4,177,302 to Earle et al. The Earle patent is directed to applying a relatively uniform hard surface coating to an article of dissimilar material by thermal liquification of the material and the adjacent portion of the article. A metallic powder is deposited on the annular seat of an engine value and a continuous laser beam is directed thereat to form an extremely hot zone. The value is rotated and the focused laser beam liquifies both the powder and an adjacent portion of the value at the interaction zone relatively rapidly without excessive heating of the entire engine valve to form an alloy of the materials. This alloy surface is relatively thick, 0.050 inch, very hard, forms an excellent interstitial bond and has a uniform distribution of alloyed materials therein.
Additionally, U.S. Pat. No. 4,015,100 to Gnanamuthu et al. discloses the use of a continuous laser beam to alloy a relatively thick metal powder coating (1/4" and greater) to a metallic substrate to improve the surface hardness thereof. The metal powder coating contains minor components of an alloy having a metal of the substrate as a major component thereof. The laser melting promotes mixing with a depth of melt of 2-200 mils.
U.S. Pat. No. 4,157,923 to Yen et al. is also directed to surface alloying and heat treating processes for purposes of case hardening of metal articles. An alloy layer is deposited on a base metal by plasma stream spraying of powdered alloy metal. A high energy beam is then directed at the coated surface to melt the deposited material and a portion of the base metal thereunder. Deposited alloy layers having a thickness of 6 mils have been melted along with the subjacent base metal to a depth of 25 mils to form the desired surface alloy.
The foregoing processes are accomplished by using a continuous beam of radiant energy (e.g., CW CO.sub.2 laser) moved relative to the surface to be alloyed. Such processes cause deep melting resulting in a very low surface concentration of the alloyed material. Additionally, relatively long melt times are required which cause dilution of the alloying specie. Such techniques are most effective when the alloyed materials are intended to be thick and hard for purposes of wear resistance. However, it would be most desirable to adapt such techniques to alloy very thin metallic coating (e.g., less than 10,000 .ANG.) of an electrically conductive material on metallic substrates to provide corrosion protection, reliable electrical contacts or the like. In addition, it is further desirable that the surface of the resulting alloy have a high concentration of the very thin metallic coating material.
Accordingly, there is a need for a method for alloying very thin films of metal with substrate materials without substantial mixing and dilution which will result in a high concentration of the very thin film of metal at the surface of the resulting alloy.